The long-term goal of this proposal is the characterization of the structural dynamics that accompany electron transfer at the active sites of heme proteins and protein complexes. This will allow us to determine the molecular mechanisms by which this important physiological process is regulated during respiration in living cells. We will employ time-resolved optical spectroscopies to follow electron transfer dynamics on appropriate timescales. Components of the mitochrondrial electron transport chain, specifically cytochrome c oxidase and bc1 complexes, will be investigated using time-resolved resonance Raman techniques already available in our laboratory. Several new classes of photoactive molecules will be used to initiate electron flow with protein systems that have been poised in specific redox or reactive states. This technique will permit us to isolate and characterize the extent to which protein conformational dynamics control the rate and specificity of electron flow in multi-component complexes.